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Do missiles need anti-icing?
Are anti-icing systems needed for any missile carried by a small combat jet? And if so, how is this achieved?
I realize of course that missiles typically fly at Mach 2.5+, so the compressive effects will heat the air around it. However, a jet is typically carrying the missile under its wing, and can be fighting at subsonic speeds. Flying through clouds or rain, or high altitude subsonic dogfight, are very ice-prone situations. If the missile has iced up and then is fired, it could fail.
Moreover, even at supersonic realms, there are other parts of the missile besides the nose and leading edges.

Top wing surface, if it gets iced, will affect the camber and lift.

Trailing edge of a control surface.

Main body could theoretically get too heavy with ice.

Seeker head could ice up at any time before firing, and if not ready, will prevent lock on.

I highly doubt the inside of the missile will be hot enough to cover all those areas, especially the nose and wings. So do missiles need anti-icing, and if so, how is it achieved? If not, why not?

Fighter jets are not built to fly in icing conditions.

@Sanchises I don't want to blow open a big discussion on that, but I will say I find that really hard to believe. I mean what about Russian fighter jets? What about the pitot-static system? Gonna ask a different Q on the latter, and maybe more.

Let me rephrase: "Fighter jets may not be built to fly in icing conditions". ;)

Related: When and where are anti-ice required for supersonic flight?

Why do you think icing would be a problem for missiles? Normally, you're looking at aerodynamic effects and sudden shedding; what's the sequence of events you see as a problem for missiles?

@fectin the question literally lists those potential issues...

Remember Ironman "So how did you fix the icing problem?". The idea of de-icing, let alone the implementation of, external equipment like missiles, wing tips & fuel tanks are classified,  The theory was that enemies who try to copy the technology might get a surprise eventually and would be discouraged from challenging the powers that be. Of course they'd need some way to de-ice to sustain aerodynamic motion, esp. those requiring accurate targets: increase temperature, reduce pressure, increase friction, reduce altitude or chemical antifreeze eg. salt.

Well, icing only happens in visible moisture, at temperatures "around" the freezing point (0 C). So first, any IR or optically guided missile cannot be fired in those conditions. (If you are in visible moisture (clouds), then the seeker head cannot see anything anyway). and second, radar guided missiles are generally fired at higher altitudes, where the temperature is well below freezing. (Air-to-air missiles, ARMs).

Also, to the comment, "Fighter jets are not built to fly in icing conditions". Not true, they have pitot heat and (at least in case of F-4), Engine anti-icing. But this is not relevant. The point is that, generally, (except for interceptors), they are not expected to FIGHT in IMC. (how can you fight what you cannot see?) And even interceptors (which do need to perform their mission in all-weather conditions), will generally be operating at high altitudes, well above the freezing level, where icing is not an issue.

In a history of NASA's Icing Research Tunnel, "We Freeze to Please," NASA SP-2002-4226, Fig. 5-2 shows Hellfire missiles being tested.  But it conspicuously mentions nothing about de-icing systems for them, unlike the many pages that it devotes to de-icing for other aircraft and aircraft parts.  So it's likely that such de-icers are either still classified, or were deemed unnecessary.
Dohn Joe's remark, that Hellfires this icy could not be laser-guided, suggests that classified is more likely.

I wonder whether such an icy Hellfire would be ready to use. Since these missles are laser guided, I would assume that a frosted over nose would block the reflected guiding laser from reaching the sensor of missle. While nowadays most Hellfires are shot from drones in places such as Afghanistan, they were initially designed as tank killers with the Soviet army in mind. Thus, in a hypothetical Ostfront 1989 scenario, there could be Apaches in icing conditions on their way to blunt the armored spearhead of the Warsaw pact. I am sceptical if iced missles as shown above would be ready to be used.

Is it possible the sudden acceleration of the missile is enough to shake free ice, particularly on the nosecone? Initial lock can be handed over from the aircraft's systems...

@SnakeDoc It would likely serve to shake free larger chunks, but the bigger concern for me would be the literal chilling effect that the ice has on the internal systems.

..suggests that classified is more likely. Aliens

Do missiles need anti-icing?
Apparently not, because they have no provision for that. At least none of the USAF/NATO missiles I've ever seen.
And it wouldn't be just missiles, but bombs as well. The arming vane on the standard M904 nose fuse has a vane that needs to spin for X seconds to arm itself.
As well as the control surfaces on smart bombs.
If anti ice were a needed thing, one would presume that it would have been designed in.

In my USAF days, I spent a few years in munitions development. I don't remember a single requirement related to icing. Dealing with cold temps were a given (-40 to -45 deg C) whether it was a high altitude loiter or sitting alert for days on the flightline in Alaska in January.

No, for mostly the same reasons that fighters don't have specific provision for such systems: altitude, weight and complexity/failure rates.
Altitude: icing conditions are only encountered within a relatively small window of approximately 3,000 - 6,000 metres above sea level. Since neither a fighter or its opponent would want (or even be able) to fight in such adverse conditions (see linked question), they would simply climb to higher ice-free altitudes - at which point their weapons systems are implicitly no longer subject to icing issues.
Weight: every weapon system is constrained by many factors, one of the most important of which is its weight. And a missile already has relatively little weight to play with (e.g. the AIM-9H "Sidewinder" is ~87kg). Adding any subsystem(s) to a missile design would either increase the weight of the missile, meaning potentially fewer can be carried and/or decreasing its performance and endurance; or decrease the amount of explosive payload or fuel, making it less effective. Either way you are now paying for the weight of that extra subsystem, regardless of the actual weather conditions the missile is being used in. Since icing conditions are relatively rare, that means in the majority of cases of missile use, anti-icing equipment would be dead weight.
Complexity: adding another subsystem to what is already a complex weapon would exponentially increase the likelihood of failure, due to interplay between the new subsystem and the existing ones. What's worse: having a missile you know that cannot operate in specific weather conditions, or one with a higher failure chance in any weather?
Even a "winter" foe such as the USSR/Russia is subject to the same constraints as above. Their aircraft and weapons systems are likely to be slightly more capable in icy conditions simply because they have more experience with such conditions, and it is possible that as a result of that experience they have been able to design lighter and/or simpler de-icing systems - but the simple reality remains that if there's a choice between fitting an entire subsystem or simply telling pilots to get above ice formation, the latter is going to be the simplest and most logical choice.
It's also pertinent to note that fighters that carry missiles internally would implicitly be more capable combatants because those weapons would not be subject to icing before launch, and after launch their speed would preclude it - but of course, that assumes that fighter is itself actually able to operate within icing conditions to begin with!

A missile only works once--it seems to me it would be possible to build a simple system based on a film over the missile that peels off when it's fired.

Missiles, at least the ones in question, do not need anti-icing as much as aircraft do.
The typical air-to-air missile has roughly the following mission profile:

0 - 2 seconds: Drop from the pylon or internal launcher
2 - 10 seconds: Accelerate with the solid fuel rocket motor
10 - 120 seconds: Adjust the trajectory with the fins to hit the target
120+ seconds: Fall to the ground or self-destruct, model-dependent

The numbers are very approximate, give or take 500%, model-dependent, but this is the general idea. An air-to-air missile is not a fixed-wing aircraft. It's more of a bullet that can turn a little (or a lot) to get on the exact intercept course towards its target.
What happens when a surface gets iced is partial loss of lift production.
Fixed-wing aircraft require L=W (lift=weight) to sustain level flight; if L<W, they will lose altitude. The wing can only produce so much lift. Increasing the angle of attack to add lift reduces the L/D ratio, so the aircraft will lose range - potentially so much that it can't land.
Fuel is the limit. Airliners, for efficiency reasons, try to carry just enough fuel to the destination, plus the mandatory reserve. Combat aircraft have to choose between fuel and payload. All in all, multiple-use aircraft have limited thrust, limited fuel, and have a destination to reach, so the margin for acceptable loss of lift is limited.
An air-to-air missile is never in sustained level flight, the way an airplane is. It may have L>W or L<W, using its kinetic energy from the initial acceleration phase to adjust its trajectory. The maximum range of AAM can be as high as 100 miles and more, but the average range they're fired at is a fraction of that maximum, for many reasons - detection, rules of engagement, effectiveness, repeat launches. In short, range is not very important.
Icing will severely degrade a missile's performance, primarily by reducing the seeker's capabilities. The loss of control surface effectiveness is a secondary, less significant effect. But the control system in any modern missile is fully computerized and relies on feedback from the INS and the sensor to judge the required control surface deflection, not just on assuming a given transfer function. In other words, if the fins are only 30% effective, they will have to be deflected more and for a longer time. The missile will have less range and less maneuverability, but it's still useful.
Half-joke, but "they don't call them hit-iles". Any missile launch being hit or miss is a given. Icing conditions will greatly reduce their effectiveness. But as a design balance, if you expect e.g. 5% of your launches to be in icing conditions, and icing to reduce the Pk by 50%, fitting an anti-icing system could make a difference between an overall Pk of 39% and 40%.
Failing an extra 1% of the time is not decisive for a single-use device such as a missile. On a multiple-use aircraft, on the other hand, that 1% failure rate would be completely prohibitive.
In short, air-to-air missiles don't get deicing because:

Air-to-air missiles have a much larger margin for loss of lift (as it's only needed for control, the flight itself is ballistic) than fixed-wing aircraft do.
Missiles are inherently single-use, and not very reliable in the first place, so rare icing-related failures don't make a big difference.

This is for AAM. Some long-range cruise missiles (AGM-109 and foreign counterparts) do have deicing, but only for the engine inlet, as it's too bulky for the wings.

I asked a colleague who worked as a combat aviation pilot in the Russian Air Force, he said that from what he knows, combat missiles (for example such as - Kh-25) are not equipped with anti-icing systems, BUT they are covered with a special anti-icing varnish.
Offtopic: Nevertheless, there are missile cooling systems for operation at high temperatures

special anti-icing varnish Would love to know more about that. A special material that doesn't allow ice to stick...that would be the "dream scenario". No bleed air, no heating elements, no de-ice boots or other vibrators... No engine power required at all to protect against the ice. That would be very useful on civilian flights, indeed on all aircraft and missiles too. A quick google search shows some claims to these things, but ive never heard of them before and am skeptical to its effectiveness.

@DrZ214 the composition of the varnish, as he said, is a state secret, but it seems like this varnish is covered during production

A lab at Fraunhofer now offers custom design of anti-icing coatings, using a bunch of techniques.  A whole set of families of unobtanium!

Remember Ironman "So how did you fix the icing problem?". The idea of de-icing, let alone the implementation of, external equipment like bombs, missiles, wing tips & fuel tanks used to be classified. The theory was that enemies who try to copy the technology might get a surprise eventually and would be discouraged from challenging the powers that be.
Of course they'd need some way to de-ice to sustain aerodynamic motion, esp. those requiring accurate targets. Some methods include increase temperature/pressure, reduce pressure, increase friction, reduce altitude or chemical antifreeze eg. salt or glycol. High speed flight can achieve some level of de-icing similar to application of anti-icing fluid with forced air.
Here's a chart showing the effects of altitude & pressure on phase points of water; it shows water can stay frozen at 100°C at 2.216 GPa, or 350°C at about 150 kbar.

Flydog update:
Increasing altitude generally gives lower surrounding air pressure. Bernoulli's effect is the reduction of pressure due to increase in fluid flow (relative) velocity, being one of the sources of lift in current aviation. The pressure inside the plane does not affect the pressure outside the plane, and therefore icing conditions, and is always greater than outside pressure in flight. In addition to lift, icing also affects pressure instruments such as Altimeters, Airspeed indicators, and Vertical Speed Indicators.
During cold flight, ice starts forming on leading edges of the plane. Jets (jet planes) typically have 'hot wings' (hot air bled from the engine compressor) to prevent icing (fighter jets typically don't fly in icing conditions or generally avoid them; refer to F-14 & F16 flight manuals).

Another method of breaking ice crystals forming is to increase pressure then reduce it (generating heat then cold around the iced area) to produce a shock (similar to that of rubber boots), as demonstrated in this volcanic flight, and is often as simple as changing altitude. The same reason results in vapor cones/ballerina skirts forming when increased localized air pressure generated by air compression from supersonic travel suddenly drops (usually behind wingtips or canard).

Air pressure can be increased by adding molecules (as in supersonic travel compressing leading air) or addition of heat eg. by 'hot wings' or generated by compressing air (from supersonic travel, as in ramjets). If your confusion stems from the chart, the chart illustrates the effect of pressure on state (& temperature) of matter; it's not a chart of altitude vs pressure.

I'm not a pilot, but... As I understand it, the pressure goes down as you go up (a commercial airliner's fuselage is basically a pressure vessel holding the passenger area at a much higher pressure than the outside).  In addition, Bernoulli tells us that the pressure on the skin of an aircraft will be lower when it's flying than when it's stopped.  I can't imagine how you'd get pressures much greater than atmospheric pressure on the outside of a plane.  For the most part, you'll be dealing with conditions lower than the triple point on that phase diagram.

@Flydog57: Air pressure can be increased by adding molecules (as in supersonic travel compressing leading air) or addition of heat eg. by 'hot wings' or generated by compressing air (from supersonic travel, as in ramjets). If your confusion stems from the chart, the chart illustrates the effect of pressure on state (& temperature) of matter; it's not a chart of altitude vs pressure.

I'm an engineer (though chemical, not aeronautical - lots of thermodynamics and all my fluid mechanics were about turbulent flow). I can read a phase diagram. My point is that atmospheric pressure is about 100 kPa. 2 GPa is about 20 kilo-atmospheres, about the pressure at the bottom of a 60 km deep ocean if one existed. At that point you picked on the phase diagram, the solid water isn't even ice as we know it, it's "Ice VI" or "Ice VII" completely different phases of water nearly no one's ever seen. https://en.m.wikipedia.org/wiki/Ice_VI

Both "Ice VI" and "Ice VII" (along with other forms of ice) are shown in the graph (with related temp & pressures they exist at). The discussion is about projectiles and icing conditions in the air; it's not about pressures greater than 1 ATM or frozen water (ice) at temperatures above 4°C


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How does the F-16 achieve negative stability?
F-16 has negative stability at subsonic conditions. How is this achieved? To me it looks like a very streamligned, very normally shaped aircraft.
Is the Center of Mass actually behind the Center of Lift? If so, how? I looked up dry engine weight as a fraction of total empty weight, and got 16%. For dry engine weight as a fraction of normal loaded weight, I got 12%.
All the weapons and ammo seem to be ahead of the body or in the middle of the wings or body. So the only thing left is fuel but I cannot imagine how you could fit a large amount of fuel very far aft in that small jet.
Is there some other way it achieves negative stability? How does the F-16 do it?

You cannot tell where CG is from just looking at it. Engine is in  the back. It weighs a bit.

The wings are shifted forward relative to a stable design. A few inches are enough already.

@PeterKämpf It sort of does, but it depends on the terminology "negative stability" and "relaxed stability". I see those two terms used interchangeably but ive always thought the first has C_g behind C_L, whereas the latter means C_g and C_L coincide (or extremely close). Correct me if im wrong.

Nothing to correct. The difference is indeed how much the cg has been moved back. But the principle is the same and the difference in cg location is small.

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How are missiles attached/detached from a combat jet aircraft?
What is the mechanism holding a missile onto the hard-point of a wing, and how does it smoothly release the missile when it's ready to fire? What about the electric cables that also need to detach without getting caught?
I just want to get an idea of how simple or complex it is. I hope it's not some hydraulic "nightmare" device.

AIM-9 Sidewinder and AIM-120 Scorpion are mounted on LAU-7 or LAU-127/8 launchers, known as "rails".
Upon signal from the pilot, the motor fires. The missile goes forward.
The umbilical cable connecting the missile to the launcher breaks away as the missile moves forward.
As far as how they're held on, there is a spring loaded lock mechanism, that is overcome by the missile motor.
pg 3-22 here:
https://www.globalsecurity.org/military/library/policy/navy/nrtc/14313_ch3.pdf

DETENT,  DETENT  LOCK,  DETENT-LOCKSOLENOID, AND SNUBBERS.—The detent, detentlock, detent-lock solenoid, and forward snubbers (fig.3-18) make up an  electromechanical system that restricts longitudinal and lateral motion of the mounted missile. This system prevents accidental launch or release during catapult takeoff or arrested landings. A slotted detent wrench/safety pin is installed through the mechanism assembly to prevent movement of the aft detent and to safe the launcher firing circuits. It is also used to raise the aft detent lug sufficiently to clear the missile  hanger  during  loading. The  slotted  detent wrench/safety pin is removed before flight. When the detent is down (normal), the striker points make contact with the buttons (contacts) on the forward hanger of the missile. The forward striker point does not have a function for the missile; the aft striker point makes contact between the missile and the firing safety switch

Body mounted missiles such as an AIM 7 or AIM -120 on a F-15 are kicked out away from the fuselage a couple of feet before the missile motor activates. There is a long cable that breaks away once the missile starts to leave.
The ejector foot is actuated by a small explosive charge.

Interesting historical sidenote... A spring detent on the AIM-9 rail keeps sidewinders from freely sliding off the rail until sufficient rocket motor thrust can overcome the spring force,
During late 70s, (or early 80s?), it happened that the manufacturer of these springs screwed up and released a large lot of them for F-15 fleet with too high a force, so high in fact that full rocket motor thrust was insufficient to overcome the restraining force. This had to be kept very secret whilst a corrected batch of replacement springs was manufactured and delivered to the fleet.

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In missiles, what actuators are used for the control surfaces?
In a normal airplane, hydraulics or even cable-pulled systems are used to power the control surfaces. In a missile, that seems infeasible. What device is used to move the control surfaces, and where does it get its power?

Battery powered servo actuators.
From www.acc.af.mil, for the AIM-120 Scorpion:

The control section consists of control electronics, actuator batteries, and four independently controlled servo-actuators.

Further on the actual AIM-120 control mechanism:
(Courtesy of the Turkish AF http://web.deu.edu.tr/atiksu/ana44/air3.html)

The Weapons Control Unit (WCU)-11/B consists of four independently controlled electro-mechanical servo actuators, four lithium-aluminum batteries connected in parallel, and a steel fuselage section that is bolted to the propulsion section aft skirt. Each actuator consists of a brushless DC motor ballscrew, an infinite resolution potentiometer directly coupled to the output shaft, and pulse width modulated control electronics. The output shaft is engaged directly to a squib actuated lock so that it does not interfere with the fin (control surface) installation and removal.

Thanks but can you explain what exactly is a servo-actuator? E.g., a DC motor powering a worm gear?

@DrZ214 - Info added.

What is a squib actuated lock, please?  Sounds like an explosive bolt sort of arrangement that... locks when fired, or unlocks when fired?  Does the lock keep the shaft (or fin) from moving until fired, or connect the actuator to the fin when fired?

@Technophile - I believe in this case it is a spring loaded catch mechanism. AMRAAM are often loaded on the jet with only some (or none) of the fins installed. They are stupidly sharp, and right at head height. Bumping into one can easily draw a little bit of skull blood. To install, you push the fin on until the catch grabs.

@WPNSGuy  thanks!  That covers fin installation but not the "squib actuated lock".  Found material at https://www.globalsecurity.org/military/systems/munitions/aim-120-design.htm.  Guessing the lock is released by firing the squib, and holds the fins lined up straight until it's time for the servos to take control.


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Is there a way to **decelerate** significantly without an "eyeballs-out" sensation?
I'm not sure how to describe this, but the problem came about as I was computing some acceleration profiles. Let's say you want to get up to Mach 3 (939 m/s) quickly. If you accelerate at 3 g's, this will take 31.923 seconds (not bad), and in level flight would cover a ground path of 14.991 km (kind of a lot, but not really comparable to typical flight lengths).
Only problem is, when it comes time to land, you have to slow down. If you want to do that quickly too, well, 3 g's in the "eyeballs-out" direction is a little hard on the eyeballs. Doing 1 g would take 3 times longer but also cover a ground path more than three times longer because of that pesky time squared in the equation for uniform acceleration ($\Delta x = \frac12 at^2$, if initial x and inital v are zero).
So you could gradually slow down over an even longer distance, or...is there some other, quicker way that doesn't involve much eyeballs out? Could you maybe fly a circle over the airport and decelerate that way, so that you feel the g-force mostly pointing down into your seat? Is there some other way you could "cut into another plane"?
The goal is to decelerate quickly so that the average flight speed (over the whole journey) is very close to Mach 3.

I'm voting to close this question as off-topic because this question belongs on Physics SE.

I really don't want to imagine someone circling the airport at Mach 3!

@DanHulme well okay, circle before they airport such that when your 360 degrees is done, you are lined up with the runway a few miles out.

How will decelerating in a spiral increase your average speed?

Just turn your seat around.

Not sure exactly what problem you're looking to solve, but F1 drivers will undergo 5G deceleration several times per lap during practice/qualifying/race. Part of maintaining a high average speed involved high acceleration and deceleration rates.

@Freeman - and F1 drivers are in peak physical condition as a result, especially the condition of their neck muscles. Not saying fighter pilots aren't, especially naval pilots who decelerate even faster when they catch the 3-wire, but it's not an everyday thing.

As far as coping with the deceleration without your head snapping forward, you could always perform a maneuver like Pugachev's Cobra, essentially nosing up suddenly into an extreme AOA and then nosing back down. That both slows the aircraft and diverts the deceleration force downward in the direction of the pilot's seat, which pilots train to withstand. However, you'd likely either rip the wings off or send the pilot's head into his lung cavity if you turned the aircraft's main airfoils into airbrakes at Mach 3.

@KeithS That's a great point. There should be some other angle of attack, however---not 90 degrees, but maybe 45, maybe less---that works at Mach 3. Still would be dangerous tho.

If you need to have an average speed of Mach 3, then cruise at a speed higher than Mach 3 and land at Mach 0.2. And decelerate at a comfortable rate.
Or fly in a cryogenic wind tunnel to get the speed of sound down so much that Mach 3 is actual a very low speed.

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What is the trim wheel behavior on the A320 family?
The Trim Wheel in the A320s usually is moving by itself, representing the current stabilizer position (or rather command). Before takeoff I believe it is set by hand to set the takeoff trim.
During flight it is not touched unless the aircraft goes into direct law or "worse".
What happens though if the pilot touches the trim wheel in normal law?
Will the aircraft shift into another Law where manual trim can be used? Will it move the stabilizer to the position set by the pilot and then adjust back to where it would usually be, possibly turning off any AP that is engaged? Will the aircraft ignore the manual input?


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On a modern Aircraft like the A320 or B737, is it possible to disable computer interference with the control inputs of the pilots?
One more question due to the recent and unfortunate events in the case of the second ever B737 Max 8 crash.
As far as I know, the first of the two crashes was caused by the MCAS System, that overwrite the inputs of the pilots as a faulty reading of an angle of attack sensor indicated a stall (that was not actually happening).
Knowing (rather, having an idea of) what a difficult time pilots have in such situations I wonder if it is at all possible to stop the aircraft from executing such maneuvers.
I know that on an Airbus A320, there is different flight laws. While during "Normal Law" all the various protections the A320 has are active (Stall protection, overspeed protection, bank protection with all their proper names and so on), they are not active in "Direct Law". There are two more levels between "Normal" and "Direct" (namely "Alternate" and "Abnormal Alternate"), but let's ignore them for this question.
I have never flown an A320 in real life and lack the training any A320 pilot goes through, but in the Simulators I have flown (not talking FSX here!) you could "exit" out of "Normal Law" by turning off the flight computers on the overhead, like the ELAC, FAC and SEC, and go into alternate or even "Direct Law".
So the question is: is this actually possible on a real A320 as well? And is there a similar method on a B737 (Max 8)?

Are you asking about disabling flight computer control, or are you asking about disabling the MCAS pitch-down on incorrectly detected high angle of attack specifically? For the latter, see e.g. Nathan Chetram's answer to Boeing 737-MAX stabilizer control.

@aCVn thanks for the link! Though obviously connected to the incidents, this question was intended to be general in nature ;)

For A320 it is possible also in the real aircraft. The problem with disabling flight control computers is that it also disables associated control surfaces. If you disable all five computers, you disable all surfaces and the aircraft goes in to "Mechanical backup" mode, which leaves pilot only the control of the trimmable horizontal stabilizer (ie. pitch trim) and rudder. Wide bodies are similar, although they have more redundant flight control computer architecture.
The correct procedure to override normal law in Airbuses is to switch two ADRs off, which forces controls to alternate law. This disables most of the protections leaving only load factor protection. All control surfaces would still be operating with automatic pitch trim. This would be used only in undue protection activation.

For those interested, Google found the procedure https://www.airlineratings.com/wp-content/uploads/articles/image2015-03-25-114958.pdf

To add for the mechanical backup, it is only true on A320, because it is the only two things still mechanicaly linked to the surface they control. On all Airbus after A320, it is all fly by wire. But if you kill PRIM and SEC on these plane, you still have control of the surfaces through the stick and rudder.

@ChrisLau The early A330/A340 have mechanical rudder. The A345, A346 and the A330s built at some point after that switched to electric rudder. There is a completely independent electrical rudder backup unit on those planes.

The more correct answer is that it is possible to disable the computer control on the Boeing 737 Max 8, but in order to avoid a crash you may have only 40 seconds in which to do so -- including the crucial first step of diagnosing the problem (which in the first accident was not obvious: a single faulty sensor indicating a stall that was not actually happening).
The pilot can optionally turn off the automatic stabilizer inputs from MCAS, an auto-stabilising feature intended only for a flight state outside the normal flight envelope (because, the B737 being aerodynamically unstable, a software routine was designed so that it would feel and behave in that situation the same as an aircraft that is aerodynamically stable).
But the pilot has first to correctly determin whether the aircraft is outside the normal flight envelope, in circumstances where he cannot rely on what his flight instruments are reporting.
If the aircraft is outside the normal flight envelope, and inherently unstable, disabling the MCAS system would be a bad idea.

Several elements of this answer don't really reflect the systems in the Max, and the post doesn't really answer the more general question as stated.

Also, the 737-8 is not aerodynamically unstable

Also the statement "you may have 40 seconds to avoid a crash" is way to broad and refers to the recent finding specific to the lion air crash.

@Maverick283  My statement that a pilot has only 40 seconds to decide is entirely correct. You imply that it is hypothetical, because it only happened in half of the cases. Since it did happen, it is a proven fact.

The computerised maneuvering characteristics augmentation system was added to the aircraft because shortening the engine mount structure tends to dangerously force the nose up, increasing the possibility of a stall: I characterise this as an aircraft which is aerodynamically unstable. The two crashes tend to prove that this is the case.

@Ed999 the crashes tend to prove that a runaway trim that is left uncorrected by the flight crew causes the aircraft's handling to become unstable - if the trim was stopped from moving i'm sure the aircraft would have been perfectly stable.

You're statement that a pilot in a 737 Max has only 40 seconds to avoid a crash is absolutely incorrect. Also, it is not 40 seconds to decide, but 40 seconds to act in the situation you probably refer to (the simulator sessions they did).

Then most glider would be unstable too! If you initiate a turn and return the stick to neutral in for example an ASK21, it will have the tendency to increase the bank angle, making it an aerodynamically unstable aircraft according to your definition.


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What's the MTOW I can fly with a PPL?
I thought there was a limit but I can't remember and can't find it anymore.
So with an US PPL, what would be the maximum takeoff weight that an aircraft would be allowed to for me to still fly it?
It'd be great if anybody could point out the appropriate FAR, too!

There are no explicitly regulated MTOW limits for a private pilot with a valid medical but extra certifications/training may be required to fly heavier aircraft.
The often cited number is 12,500 but this is somewhat incorrect depending on how you look at it. You will need a Type Rating to fly anything over 12,500 but strictly speaking those can be added to a private pilots license.
If you are a PPL flying under Basic Med you are limited to 6000 LBS MTOW

14 CFR § 61.113 - Private pilot privileges and limitations: Pilot in command.
(i) A private pilot may act as pilot in command of an aircraft without
  holding a medical certificate issued under part 67 of this chapter
  provided the pilot holds a valid U.S. driver's license, meets the
  requirements of § 61.23(c)(3), and complies with this section and all
  of the following conditions and limitations:
(1) The aircraft is authorized to carry not more than 6 occupants, has
  a maximum takeoff weight of not more than 6,000 pounds, and is
  operated with no more than five passengers on board; and

Just a note on practicality, even if you don't get into the 12,500 range you are likely to need high performance and complex ratings to fly anything much bigger than a 172. While there are some big fixed gear aircraft out there most of your common 6 seat planes (Saratoga, Bonanza, etc) are retracts.


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A321XLR pair of bulges on the belly
What are these two elements of A321XLR? Are they structural elements or something else?

They are not present in A321 or A321neo models:

Pics credits: Nathan Walkowiak (CC-BY), Jean-Pierre Nivet (CC-BY), Clemens Vasters (CC-BY)

It's the fuel pump fairing for the Rear Center Tank which is only there in the A321 XLR.
There's one on the underside of the wings too (on both A320 and A321, neo or XLR)
The round bulge at the root is what connects this wiring to the fuel pump which is connected to the tank and the two "rods" are covers for the wiring of these fuel pumps. They are outside to prevent ignition sources.
Image of this thing in the A320ceo

I believe it's the same thing but it looks different in the A321 XLR.


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Why east-bound flight is faster than the reverse flight along the same route?
Compare
http://www.wolframalpha.com/input/?i=Alaska+Airlines+764+yesterday+duration
and
http://www.wolframalpha.com/input/?i=Alaska+Airlines+771+yesterday+duration
Why the east-bound flight is about 20% faster than the west-bound reverse flight?
Is it because of the average airflow is east-bound?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Does ground clutter exist over the ocean?
Ground clutter is a radar term that basically means a bunch of noise, or cluttered returns, that occurs when you point the radar down at the ground (like for a radar patrolling aircraft). The terrain reflects radar and you get a big blur.
Today, ground clutter is not a problem because we have great computers that can filter out the static ground returns, and only show the moving returns. However, I want to ask if ground clutter needs to be filtered when over the ocean.
The ocean is pretty flat, but always has at least some small waves. Do small waves produce any ground clutter? In other words, does ground clutter exist over the ocean during calm seas?

From personal experience on the Atlantic I can assure you that given the right amount of wind, the waves do not qualify as small.

@DeltaLima I meant "small" as in relative to typical terrain with all its trees, rocks, and hills.

During calm seas - no, by definition. Can't see the point in this qualification.

Sea clutter exists when you're at sea level, on a boat:  e.g. https://www.furunousa.com/Learning%20Center%20Documents/RadarTargetDefinition.pdf

@DeerHunter "calm seas" does not mean a perfectly flat ocean, because first of all, there is no such thing as perfectly flat, and second of all, because there are always at least some small waves on the ocean. "Calm seas" just means "no violent waves". It does not mean "no waves at all".

Yes, ground clutter does exist over the ocean. It depends on the smoothness of the water surface, radio frequency, antenna characteristics and atmospheric properties whether the radar will actually receive reflections from the sea surface.
A perfectly smooth sea surface will not generate clutter, but the combination of small waves and a strong inversion layer (temperature getting higher with altitude) will cause ground reflections, even for surface based radars looking above the horizon. The inversion layer will cause super refraction - the radio wave bends towards the ground / sea - and will cause a reflection off the waves which is received by the radar.
In an extreme inversion layer a so called atmospheric duct may form in which the radio signal makes multiple bounces of the sea. At each bounce part of the energy is reflected, resulting in multiple ground clutters at regular distance intervals.
Ground clutter is not always perceived as a problem. In some cases its absence may help to identify objects of interest. For example, in weather radar the absence of ground clutter behind a cloud indicates that the cloud has absorbed all the radio energy which is an indication of its severity.

source: Airbus Flight Operations Briefing Notes, Adverse Weather Operations (PDF)

Ducting is not about bounces off the surface, it is about refraction. While energy always leaks from the duct and reflects off the surface, it is a second-order effect.

Yes. That's one of the reasons why sea-skimming missiles exist: they hide their radar signature in the clutter by flying a few (dozen) feet above the waves. (They also take advantage of the horizon's blocking to reduce maximum detection range.)


text:
Can wing-tip vortices be reduced/eliminated with a rear-facing propeller near the wing-tip?
Let's say we have a propeller-driven aircraft in the pusher configuration (rear-facing propellers). If we could somehow put an engine at the wing tip (ignoring structural concerns), and maybe make the propeller rotate in the opposite direction of the way the wing-tip vortex wants to spin, would this eliminate or at least drastically reduce the wing-tip vortex?

It's been tried before.

@fooot a fascinating aircraft that seems to imply the answer is yes. However I envisioned the propellers to be a little proportionally smaller and the wing shape to be much more conventional. Hopefully the vortex-cancelling effect will be the same.

Vmca on that aircraft had to be pretty high!

@RalphJ I think the idea was specifically to have more lift and control at low speeds by using the propellers to boost the wing efficiency. The article suggests that was actually achieved.

That might actually be an interesting design to revive for hobbyists. It looks to me that it was designed to optimize space usage (hangar and deck) and robustness for carrier operations. I think there are people who'd like to have a compact and robust design. Although I kind of wonder what happens if you have engine or propeller problems...

@VilleNiemi  All well & good, but Vmca is about maintaining lateral control with a failed engine.   Small rudders + long moment arm on the engines seem to combine to give a very high Vmca, one might think.

@RalphJ LOL I guess we were actually wondering about the same thing then. (see my previous comment.)

Yes. From an aerodynamic standpoint it makes some sense to use the wingtip vortex in combination with a propeller.
In this article by Sinnige et al., the researchers modeled and tested a propeller in various positions along a semispan, concluding that there were real gains to be had due to the increased span efficiency. Up to 15% less drag was measured when the propeller was positioned at the wingtip compared to a conventional mounting:

By positioning the propeller at the tip of the wing, the slipstream interacts with the flow around the wingtip, thus affecting the roll-up and downstream behavior of the wingtip vortex. PIV measurements downstream of a propeller–wing model showed that this leads to a reduction in overall swirl with inboard-up rotation, for which the swirl in the slipstream is opposite to that associated with the wingtip vortex. At the same time, the system performance was found to improve due to a reduction of the wing induced drag, leading to the conclusion that the decrease in swirl causes a reduction in downwash experienced by the wing.
Apart from the change in drag, the interaction of the wing with the propeller slipstream also modifies the wing lift. The locally enhanced dynamic pressure increases the lift over the spanwise part of the wing washed by the slipstream, which is amplified by the induced swirl for the case with inboard-up rotation. As a result, a strong spanwise variation in lift occurs with the propeller on. The induced velocities caused by this lift gradient lead to a spanwise shearing of the slipstream. With outboard-up rotation, the swirl in the slipstream acts to locally oppose the increase in wing lift due to the propeller-induced dynamic-pressure rise. Compared to the inboard-up rotation case, this leads to a reduction in wing lift at a given angle of attack, thus also a reduction in maximum lift coefficient. Furthermore, the direction of the spanwise shearing of the propeller slipstream is inverted on both sides of the wing.
To quantify the potential aerodynamic benefits of the wingtip-mounted configuration, a direct comparison was made with a conventional configuration, with the propeller mounted on the inboard part of the wing. The increase in wing lift due to the interaction with the propeller was 1 - 4% smaller for the wingtip-mounted configuration than for the conventional configuration. For the latter, the enhanced dynamic pressure and swirl in the slipstream acts over double the spanwise extent, and on a part of the wing where the section lift is higher than for the wingtip-mounted configuration. At higher angles of attack, the lift advantage for the conventional configuration could be further enhanced
  by the local angle-of-attack increase in proximity of both sides of the nacelle.
In terms of drag performance, on the other hand, the wingtip-mounted configuration showed superior performance. At a wing lift coefficient of $C_L = 0.5$ and a thrust coefficient of $0.09 < C_T < 0.13$, the drag reduction amounted to about 15 - 40 counts (5 - 15%) compared to the conventional configuration. The aerodynamic benefit of the
  wingtip-mounted configuration further increases with increasing wing lift coefficient and propeller thrust coefficient, leading to a drag reduction of 100 - 170 counts (25 - 50%) at $C_L = 0.7$ and $0.14 < C_T < 0.17$. An analysis of the wing performance confirmed that this drag benefit is mostly due to a reduction of the wing-induced drag. Compared to the conventional configuration, a relative increase in span efficiency of up to 40% was measured for the wingtip-mounted configuration. Although the exact drag benefit will be specific to vehicle design and operating conditions, it is concluded that the interaction between the propeller slipstream and the wingtip vortex leads to a clear drag reduction for the wingtip-mounted configuration.

Keep in mind this is purely aerodynamical, of course, and from a structural standpoint things might not be so clear-cut. While weight in the wings helps alleviate the bending stresses generated by lift, a large mass at the tip can lower the first bending eigenfrequency to unacceptable levels, risking a coupling of the wing oscillations with some other mode in flight or on the ground.
As far as I am aware wingtip fuel tanks have mostly been used on relatively low aspect ratio wings, which lends some credence to the magnitude of this problem.

Well done for spotting that paper! Also, putting the propeller before the wing makes a ton of sense. An additional problem with having this weight at the wing tip: Landing. Passenger aircraft must be able to land at constant 3° glide slope (in case the pilot doesn't pull up in time) -- with nice slender wings, that means unless your engine is very light, you have a problem. So I would still not expect to see this in practice, at least not without a host of other smart ideas to make it work.

That paper had been sitting in my "to read" pile for half a year, this was more of a happy coincidence. Also, good point about wingtip clearance; there is also the issue of FOD damage for engines hanging outside the paved width of a runway.

Eliminate, no. The wingtip vortices are inherent part of lift generation. There is no lift without wingtip vortices. The wake vortices are carrying the momentum that was given to the air to produce the lift, and to cancel them, you'd have to give the air the opposite momentum, which would negate the produced lift. See
How does an aircraft form wake turbulence?
Improve efficiency, yes, a little bit. The propellers would cause downwash in their span outside of the wing tips, effectively extending the wing span and longer wing span means, slightly, less induced drag.
However, such design would have extremely poor single-engine handling, not only because the thrust would become very asymmetric, but also because the lift would partially depend on the engines and therefore the side with stopped engine would also loose some lift, and deflecting aileron to compensate would generate more drag to make the thrust even more asymmetric. And it would have poor all-engine-out performance too due to the loss of lift. Does not sound like optimal approach when similar benefits can be obtained without all these problems by using longer wings.

"The wingtip vortices is what generates the lift."  Wrong, wingtip vortices are a by-product of lift generated by the wings, but they generate drag by themselves.  Otherwise why add winglets?

I agree with Micheal Hall. Otherwise, why not put some vortex generators on the wingtip to increase vorticity? Would putting propellers behind the wingtips to cancel the vortex out destroy the lift?  The wingtip vortices are a side effect of having a lifting vortex in your wing (precisly: spanwise change of strength of the lifting vortex) -- which itself is as much a mathematical construct than an actual thing.

Correct: single-engine handling would be horrible (and also putting such masses at the wingtips is a recipe for aerolastic catastrophes), and ailerons interacting with propellers must be a nightmare in terms of stability and control (and the propeller, too)  -- but the propellers wouldn't generate very much lift. Also: making the wings longer isn't always an option.

In case the wing-tip engine is providing a substantial amount of thrust, the problems indicated by Jan Hudec and Zak hold. For a distributed propulsion system, such as the configuration used by the NASA X-57, many of these drawbacks are prevented.

@MichaelHall, wingtip vortices are necessary part of the phenomenon of lift generation. There cannot be lift without wingtip vortices (more properly, without wake vortex sheet, which then composes to wingtip vortices), and at least one way of analysing the flow uses the vortices as explanation. Winglets are installed to move the wingtip vortices a bit further apart, which makes the overall process a bit more efficient, when it is not practical to do so by making the wing longer for other reasons, because aerodynamically that is always better.

@Zak, if you cancelled the vortices, you would indeed cancel the lift, but fortunately the propellers won't do that.

@MichaelHall, by the way, if you don't accept the vortices generate lift, they don't generate drag either—the aft component of the aerodynamic force on the wing does. After all, the vortices are behind the wing and no longer interact with it in any way. Of course when using vorticity to describe the situation (see lifting line theory) the bound vortices around the wing are the key part, and they generate both lift and drag.

@Jan Hudec - If you had a wing that spanned the entire width of a wind tunnel you could have a positive AoA and produce lift without vortices.  Vortices exist because in real life because the wing has a free end, and high pressure air beneath spills up and over the top.  You are the first person I have ever heard staking a claim that this was a beneficial effect that actually produces lift vs drag.

@MichaelHall Wouldn't the high pressure air already have delivered an "opposite reaction" to the wings downward push by the time it goes back on top of the wing to meet with the low pressure air? This answer shows some images of vortex wake that are causing effects long after the air craft has passed: https://aviation.stackexchange.com/a/49357/31421

@YAHsaves - Yes, it would have.  That is where and how the lift is produced.  Spilling over into a vortice at the wingtip doesn't produce any additional lift though.

@MichaelHall, if the wing spans the entire width of the wind tunnel, then the wind tunnel indeed immediately stops the vortices. The bound vortex must (and does), however, still exist, it just hits the wall instead of spilling off as the horseshoe vortex.

@MichaelHall, spilling the vortex at the wingtip doesn't produce any additional lift, indeed. It does not produce any additional drag either. Both were already produced by the bound vortex. However, the spilling vortex is carrying away the momentum needed to produce the lift and if cancel it, you have to apply the opposite momentum, therefore cancelling the lift as well…

@JanHudec You are correct in saying lift cannot exist without vortices. Michael and Zak are also correct in saying wingtip vortices do not contribute to additional lift. Wingtip vortices are what contribute to the induced drag, a finite-span 3D phenomenon. In infinite span, induced drag would disappear and you would get the 2D result, and you will still have bound vortices.

@JanHudec - I see you edited your answer from what I quoted in my first comment.  I have canceled my downvote.

@Jimmy, actually, the vortices don't contribute any additional drag either, because they are already past the wing. The lift and induced drag are both applied to the same point. Also with infinite span, you only get zero induced drag if you generate finite lift over it, which means you generate zero lift per unit of span. If you generate non-zero lift per unit of span, you get non-zero induced drag no matter whether the span is finite or infinite.

@ Jan Hudec: Vortices and drag:  The vortices contain moving air, right? That means they contain kinetic energy, right? So, where did that come from, and who keeps supplying it? -- you're not completely wrong, though: The propeller could only counter the vortex (which is mathematically infinite, decaying with inverse distance) by applying swirl (not the same as vorticity) to the central part of the vortex, making it dissipate quicker and re-using some of its energy. So no, you won't eliminate the vortex. However, the vortices are unavoidable for finite wings, not necessary.

@Zak, the vortices contain energy, which came from drag, and momentum, which came from lift. Therefore they “cause” drag and lift in exactly the same way. You should either consider them causing both or neither, but not one only….

@JanHudec If you look at the derivation of the lifting line theory, for elliptical distribution, induced downwash is inversely proportional to span. In the limit where the span becomes infinite, induced downwash vanishes, unit lift remains finite (of course, total lift goes to infinity) and is a function of the 2D airfoil lift slope and free-stream incidence only. I can't process the derivation in a comment. But if you form a question, I'd be more than happy to show my reasoning. Of course, this is just academic; for all intents and purposes, we live in a finite span world.

@JanHudec "momentum came from lift". Lift is an upwards force -- it should cause downward momentum change in the flow. However, the momentum in the vortex is mostly around the center, in circles. What the vortex does is turn the flow seen by the wing close to the tip downwards, thus reducing lift because the wing can't turn it as much anymore.You may be confusing the lifting vortex ("in the wing") with the wingtip vortex ("lifting vortex where wing ends"). This is why long slender wings are so efficient: Use a longer lifting vortex, and a smaller part of the wing affected by the vortex.

@Zak, the wake vortices move down as a whole, which corresponds to the downward momentum of the wake from lift. That is because the vortices form due to the wake behind the wing moving down and the air outside the span not moving. Momentum being vector quantity, the momentum of the circular motion sums to zero and is not relevant for this analysis.

@JanHudec I don't think the comment section is a good place to discuss this. I'm very certain of what I'm saying here. I took my aerodynamics exam twice because I got some bits quite wrong the first time round. The whole vortex thing is only accurate in potential flow, which is a pretty easily misunderstood abstraction in the first place. I have no time to write a book here but I suggest you have a look at the Wikipedia article on lifting line theory, and the "horseshoe vortex" one -- the circulation around the profile ("lifting vortex) creates lift; the trailing vortices reduce lift.

@Zak, that's correct. But my point is that whichever way you look at it, you can't cancel the wake vortex without cancelling the lift as well.

@Zak, I also think the situation is symmetrical for lift and induced drag—the bound vortex is what creates lift, but you already need the energy, i.e. the induced drag, to create it, and the wake is carrying away both the momentum and the energy. That might not be according to the lifting line theory though; I am relying more on basic laws of motion in this regard.

@JanHudec I invite you to take a look at my answer to this question. Wingtip vortices are a by-product of lift, not the other way around.

@Jimmy, yes, I am not disputing that. But they are a necessary by-product. You can't generate lift without generating wingtip vortices. You also can't generate lift without generating induced drag for reasons of plain old conservation of momentum and energy, with vortices just happening to be how these are satisfied. And, most importantly, you can't undo that induced drag without also undoing the lift.

@Jimmy, the naming of by-product and cause and effect are a bit arbitrary in this situation actually. The effects are all tied together, but the way the laws of physics are interconnected, you can explain why they are tied together starting from different points.

@JanHudec You also can't generate lift without generating induced drag for reasons of plain old conservation of momentum and energy. I don't know where you got that from. The circulation theory of lift, including lifting line (from which the term induced drag was first derived), is based on conservation of momentum, mass and energy. It clearly predicts that the longer the span, the less the induced drag per unit lift. The theory is so successful that it's used in modern aircraft design.

@Jimmy, and where did I say that you can't reduce the induced drag? I only said you can't remove it completely.

@JanHudec Yes, you need the bound vortex -- which is why it's helpful to make more use of it by having e.g. longer wings, before it goes away (more lift, same wingtip vortex!). With infinite wings (or a profile between parallel walls) there isn't even wingtip vortex. And that's the key: You don't need the wingtip vortex. It's unavoidable on finite wings, not necessary. So anything you do to that vortex later, e.g. putting a propeller in it, will not reduce lift. (Unless of course, if you point the propeller downwards or something)

@Zak, what you do to the vortex later will not affect the lift applied by the bound vortex to the wing. However, if you want to actually cancel it and leave no vortex behind—as opposed to pushing it further apart, which does of course improve efficiency—you have to take back the momentum of the downwash. So it will get the opposite force to the lift applied to it, and if it is part of the same aircraft, it will undo the lift (convert it to tension and torque, not nothing at all).

This thread is becoming ricidulously long. I suggest you look at  @AEhere's answer and the linked paper there: Fig 24 nicely shows how lift is almost unaffacted by moving the prop to the wingtip, but drag is nicely reduced. There's more figures showing how the vortex is weaker if the prop is turned on.  I'm tapping out now.

Doug MacLean has a great synopsis of wingtip devices that is worth reading, and gives a good intuition as to why propellers could reduce, but likely not eliminate, the wingtip vortex.
To summarize: the wingtip vortex (really, the rolled-up vortex wake sheet), creates a downwash at the wing which effectively reduces the angle of attack of the wing relative to the freestream.  This downwash increases with the strength of the wingtip vortex, which is proportional to the lift of the wing.  The induced downwash angle $\alpha_i$ can be compactly given as $\alpha_i = \frac{C_L}{\pi e AR}$.
To compensate for this downwash, the plane has to fly at a higher angle of attack to achieve some amount of lift.  A component of the lift ($C_L\sin\alpha_i$) is now pointing in the streamwise direction. This is induced drag, and if the $\alpha_i$ is small you get the conventional induced drag expression $C_{D_i} = C_L\sin\alpha_i = \frac{C_L^2}{\pi e AR}$.
So how does this change if we add another source of vorticity? It won't reduce the lift of the wing; the vorticity generated by lifting surface will still be present. To remove the induced drag ideally our new source of vorticity will have opposite sign and equal magnitude, so as to create a corresponding upwash at the wing, which would rotate the lift vector back towards the freestream direction.
Can the propeller do that? It's worth thinking about where the vorticity (swirl) comes from in the propeller wake.  Swirl is the tangential velocity imparted to the wake; it's essentially due to viscous losses in the blades.  Well-designed propellers try to, among other things, minimize this type of loss.  If you had a propeller well-designed to provide forward thrust, you would need to put in a huge amount of energy that the viscous losses counteract the entirety of the wingtip vortex; likely much more than you need for forward thrust. In a typical jetliner wing, the lifting force in cruise is something like 20x the propulsive force.  Swirl losses, like tip vorticity, are proportional to the thrust produced by the propeller, which is an order of magnitude smaller than the wing lift.
In theory you could design a propeller to add more swirl to the wake, counteracting the tip vortex more efficiently.  But this would reduce the efficiency of the propeller at creating forward thrust, so is unlikely to give a net benefit.
As the testing shows there is some benefit to wingtip mounting. You can't design a propeller with no swirl, so you might as well get some credit for it by putting it at the wingtips (if you can accept the weight and OEI tradeoffs associated with it). But it's not going to entirely (or even substantially) eliminate induced drag.
TL:DR Propeller swirl and wingtip vorticies are both due to inefficiencies in the two systems. These inefficiencies can cancel to some extent, but they are both proportional to the amount of force being produced by the wing/propeller. Since the wing is producing ~20x the force of the propeller in cruise, the effect of its inefficiency will dominate.

Welcome to Av.SE!

The wingtip vortices carry some energy, and not leaving it behind seems like a good idea. That's why winglets are a thing, after all.
So, what if you put a propeller at the wingtip, aligning the axis with the vortex core?
The propeller would "see" the incoming vorticity, and the blades would get accordingly larger local incidences, thus generate more forwards force -- a little like putting guide vanes in front of it. Alternatively, the propeller could rotate a little slower or reduce blade incidence a little to get thrust back to where it was.
The swirl which the propeller produces would be accordingly reduced, so there'd be overall less vorticity and swirl in the flow behind the aircraft.
That all sounds pretty good so far.
However, there are a few drawbacks, from least to most severe:
1: The wintip vortex is pretty strong at its core, but angular velocity reduces quickly as you move away -- this means that the innermost section of your propeller gains most additional incidence, but since it's also moving slowest and the blades are thickest, it doesn't produce a lot of thrust anyway. The outer bits won't see much of an effect, as their own circumferential speed will be much higher than the vortex at that position
2: When mounting the propeller directly behind the wing, the blades will pass through the wing profile wake on the inboard side, where airflow is significantly slower. In the worst case, if the flow on the wing separates, propeller blades could go through a significant zone of "dead water", which means less thrust, and more mechanical load on the blades. Also more noise. Most existing pusher configurations have the propeller mounted at some distance from the wing to reduce this effect. But if you did that with wingtip pusher propellers, it would just make reinforce an even worse problem...
3: The wingtip vortex doesn't align nicely with the trailing edge of the wingtip, let alone the propeller axis. Depending on flight condition, the vortex will be stronger or weaker, and at large incidence, it becomes more of a smeared-out vortex sheet -- imagine lots of small vortices released from points along the outer wing edge, propagating in downstream direction. This means at many flight conditions you'd have the vortex not really matching with the propeller, this diminishing the desired effect, but in some other conditions, you'd have a strong, well-focussed vortex hitting the propeller somewhere off-center, and having your propeller blades rotate through that causes bad vibrations, and might also separations on the blades, which means lost thrust and horrible noise, coupled with either having to reinforce the entire drive train (or facing much higher wear and tear). You could make a propeller aerodynamically more robust to such things, but that will always come at the expense of efficiency, and that's what we set out to gain in the first place...
Pusher propellers are not very efficient to begin with (because putting the whole aircraft into the swirl coming from the propeller is still more efficient than exposing the propeller to the wake of the aircraft), and mostly used for stability reasons (This has to do with the pitching and yawing moment produced by a propeller in inclined flow) -- so although aligning a propeller with a vortex, in isolation does indeed make sense, there are too many real effects keep this from improving efficiency over a regular old front-mounted prop.
So, can't we do anything with that vortex?
Oh yes, you can! You could place a little wing inside the upwash just outside the wingspan, also known as "increase the wingspan" -- more wing makes more lift, but the vortex does not get stronger. This is why sailplanes have such long, thin wings.
Or, if you can't make the wing any longer (wing root bending moment too large, size restrictions...), add the little bit at an angle! The classical vertical winglet works like this: It redirects the inwards-headed flow above the wingtip to go straight downstream, and this produces mostly an inwards-facing force but also a forward facing component ==> this means it does exactly what the propeller can't do efficiently, which is weakening the vortex and deriving a little forwards force from it. These days, most winglets are some sort of blend between a wingspan extension and the classical winglet.

How would a turbine engine placed on the wing tip to receive the tip vortex in its entirety?

@Muze: I typed "Turbofan vortex ingestion" into duckduckgo, and wouldn't you know it, this is the first result: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750018896.pdf  -- somebody testing what happens when a wingtip vortex gets into an engine. It's not good. Stall and surge margin are reduced, and so is efficiency -- unless you hit exactly straight on, but you virtually cannot. Ground vortex ingestion is also a longstanding problem for turbofan engines: https://aviation.stackexchange.com/questions/21219/how-does-this-vortex-form-inside-a-jet-engine/21229

"Pusher propellers are not very efficient to begin with" – are you sure? Most evidence points to the contrary.

@Peter -- I have no proper literature to hand right now, but the fact that very few aircraft use pusher props (and that most pusher props are seen in photos from the Age of Experimentation) does point that way. The way I learned it: Pusher props avoid penalties for having the fuselage sit in the accelerated, swirling air from the prop, but degrade prop performance, which is (usually) worse. They're also audibly louder (can confirm from personal experience). If they are used, it's mostly due to stability reasons or to clean up the flow on the rear. Reality is likeley more complex than that ...

You can't stop them from forming, but in the case of  rotor blades you can reduce the tip vortices to the point that they are almost gone by the time the next blade arrives.


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What does "composite panels on a frame" mean?
Reading about composite materials in the 787 on Wikipedia, I came across this:

Carbon fiber, unlike metal, does not visibly show cracks and fatigue, prompting concerns about the safety risks of widespread use of the material; the rival Airbus A350 uses composite panels on a frame, a more conventional approach, which its contractors regarded as less risky.

I bolded the curious part. What does "composite panels on a frame" mean exactly?

Traditionally, aircraft are built of frames (analogous to a person's skeleton), as shown below, and the smooth outer skin of the aircraft consists of panels fixed to these frames.  There are many frames and, for example, the small size of the windows is because they have to fit between the frames.

Image source: Wikipedia
The A350 uses this traditional construction, except the frames and panels are built of composite materials (carbon fibre reinforced plastic), rather than aluminium. However, the fuselage of the Boeing 787 seems to consist of a composite "tube", with reinforcing ribs running the length of the plane. There seem to be many fewer "ribs" running around the fuselage in this design than in traditional ones.

Image source: Wikipedia

There seems to be a large spider inside that SSC...

That's a good picture of the 787 structure, but it only shows two frames for some reason. The spacing of frames and stringers is larger than planes like 777/A350 but not dramatically so.

Why is there a spider on your bulkhead?

@DrZ214, why shouldn't there be?

Composite material are built in panels. The idea is to built a metal structure (spars and ribs for example), and to cover it with composite sheets, instead than aluminium.
Why not to do a structure in composite?
Composite materials are very complex and still not completely known: we have been using Aluminium in the last century and we know EVERYTHING about it, also very unusual or peculiar behavior.
Composites are still to be completely understood, that's one of the reason to use a metal structure. Moreover composites are not suitable for isotropic stress and they will never completely replace the metal.

Saying we know everything there is to know about AL just seems to be asking for trouble...

@FreeMan indeed it's some decades without structural failures in aircraft due to the design. Because we know everything about the behavior of Aluminium, in every conditions of temperature, humidity and fatigue in daily use.


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Has there ever been a diesel-electric helicopter?
Occasionally I come across the diesel-electric (DE) design for submarines and freight trains. They say something like "a diesel engine powering an electric drive lets the two operate at their most efficient RPMs, and the electric drive has very little losses in efficiency."
So the question hit me: Has there even been a DE helicopter? Is there something about the DE system that makes it bad for the aviation industry? Weight maybe? But note we do not need a bunch of batteries like on a submarine. We don't need to store the power, we just convert it to electricity which goes immediately into the electric drive for the rotors.

"Occasionally"? All diesel-powered submarines and almost all diesel railway locomotives are diesel-electric.

@DavidRicherby Yes, these days they are. What I meant was, occasionally I read about them again.

I would guess "no" (although there are concept designs) because the technolgoy to make the batteries light and robust enough has only recently been devleoped.

@Simon If you read the last two sentences in the OP, a DE helicopter does not need any batteries because we don't need to store the power. The power immediately goes into the rotors.

If the power from the diesel goes immediately into the rotors then what is the point of flowing it through an electric system?

@DarioP See my answer. Electric systems are just a very fancy gearbox if used without energy storage.

@DarioP the idea was to let the diesel rotate at its most fuel-efficient RPM.

Maybe if you wanted to have one engine-generator power lots of rotors? SHIELD helicarrier or Robur style. Although anything big enough for this to make sense should probably use a nuclear reactor, not a diesel engine, for power. (Not claiming this makes sense, just that it would probably have to be nuclear carrier sized for making sense to be possible.)

are you referring to Otto cycle based engines or all engines and cycles fueled by diesel?

@jCisco I'm referring to a traditional piston-driven diesel engine, which does not require spark plugs because the compression ratio is so high, it auto-ignites the fuel. This was the original meaning of the word "diesel", but today it could refer to only diesel fuel, so there is room for confusion.

I think the RPM needed for an helicopter's rotor is quite constant. Thus, you can design the gearbox to operate your diesel engine near its optimal RMP when your rotor is at its nominal RPM with only one gear-ratio.

@Sanchises: A very fancy, much-more-efficient, infinitely-variable gearbox.

Helicopters currently use lightweight but fairly inefficient gas turbines. Diesel-electric engines have great energy efficiency, in terms of mechanical energy delivered per kilo of fuel. However, they're quite heavy. This means that you need more lift, which in turn means more power is needed. This negates the initial advantage.
With the advent of lightweight materials, the latter effect decreases so this situation might change.
Diesel engines can throttle up and down quite a bit, unlike gas turbines,  but this capacity is irrelevant to helicopters. Note that helicopters need a pretty constant amount of lift.

"Note that helicopters need a pretty constant amount of lift. ", do you have a reference for this? It seems to me that helicopter need to vary their lift quite a lot (for example to achieve an upward acceleration?)

@ROIMaison: That's actually not a lot of variation. You typically don't climb at 2g in a helicopter, and you certainly try to avoid descent in freefall, so the trust you need varies by less than 100% from nominal. Cars often do accelerate at power levels several times what's needed for a street-legal speeds (which is why top speeds are so much higher). IOW, cars normally just battle air and rolling friction (not much) whereas helicopters normally battle gravity (much more).

Note that Airbus Helicopters is working on a diesel helicopter, but with a direct drive, not diesel-electric.

The main advantage of diesel-electric is that it offers a great deal of torque at low motor rpm [from Physics Forums]. In effect, the torque curve of an electric motor is highest at 0 rpm, and has a very flat decrease as rpms increase.
This is of great value for vehicles that have to pull a great deal of weight, and to overcome the inertia of moving it from a standing start. It is of little or negative value for aircraft because it is much less efficient. Anytime you generate power from one medium and then use that power to drive another medium, a lot of power is wasted. So converting diesel oil to mechanical energy, then further converting it to electrical energy, is far too wasteful and unnecessary for a helicopter, where every erg counts. With helicopters, which these days mainly use gas turbine engines, as much of the power as possible gained by burning fuel is converted directly into rotary motion. Note also that massive torque at low rpm is useless for rotor aircraft, since it is only at high rpm that such craft are able to fly.
Important, too, is the fact that dual-power systems like diesel-electric add a lot of weight, since you are dealing with two different types of engine. For any aircraft, especially the helicopter, that may even be a bigger drawback than wasted energy—though these things may be viewed as two sides of the same coin.
This just in: The Economist magazine now has published an article, Hybrid power will make helicopters safer and more productive, on the advent of helicopters that will use hybrid engines.

It's also worth noting that in a diesel-electric locomotive or submarine, weight is often a good thing, since in either case if the diesel-electric power train and drive train didn't weigh enough it would be necessary to add ballast [a locomotive that's too light won't be able to pull very much, and a sub that's too light relative to its volume won't be able to submerge.

Where do you still use ergs as a unit of energy?

@immibis: In the Land of Metaphor and Colorful Writing.

@Robusto electrical engine dynamics confound simplistic models as presented. Contrary to generalization indeed
eg shuntfield DC motors run at a constant RPM display a torque curve that behaves to the increase of RPM in the following manner.  As RPM moves from 0 toward ~90% of the ideal and therefore targeted RPM.  The Torque available is 300% of that available when at 100% RPM.  Over the last 10% increase in velocity the Torque decreases to the 100% level.  That graph Torque % (x) is a line straight up then bending to the left in 10% of rotation increase =y.

Diesel-electric is chosen as a solution to a set of problems that are not really RPM related.  The primary reason in the case of trains the choice was driven by the inability to effectively use mechanical linkage to drive a number of wheels.
There is a limit to the amount of power and torque that can be transmitted via a transmission before it becomes impractical to impossible. Trains with 2000HP diesel engines require a transmission that is excessive in size and weight in order to transfer 100,000lb feet of torque to the wheels.  Further complicating this is the need to split the output accross multiple axles.  It is complicated, heavy, unreliable and inefficient, and this was apparent on very early diesel engines despite not matching the steam engines they were replacing.
The solution to gain better tractive force was to drive multiple electric motors with each motor powering a single axle (sometimes more than one). This necessitated replacement of the mechanical transmission with a generator/alternator and the necessary power handling equipment.
There are of course some overhead losses in the order of 25% due to the conversion process of mechanical to electrical plus the need to power ancillary systems such as heat exchangers, charge battery banks etc , However the increase in flexibility, reliability, performance and simplification make the overhead a cheap tradeoff compared to alternatives.
Train and helicopter specifications:

A typical locomotive prime mover is 4,400 HP and 21,000 ft lb of torque operating at 1,100 RPM. The Generator/alternator will provide 2.1MW in electrical energy. The entire locomotive weighs 400,000lb. The electric motor within a typical set of six will have torque at around 6,200 ft lbs.
Size: (Prime Mover, Generation and Drive gear): Massive.
A light helicopter such as the R22 has 124 hp and 241 ft lbs of torque at 2700 rpm  MTOW 1,400 lbs.
Size: (Powerplant & transmission) A large suitcase.
A heavy helicopter CH-53E has three engines delivering 4,380shp @ 14,280rpm with ~1,610 ft lbs of torque driving through multi-step transmission delivering nearly 600,000ft lb of torque to the main rotors shaft. MTOW 73,500 lb.
Size: (Powerplant & transmission) 10,000lbs in the volume of the mini cooper.

Bottomline

A locomotive requires immediate high torque to overcome the weight of itself and resistance from the load. The time to begin movement is critical, the train must be able to begin motion on demand. Whereas helicopters have time to spool the engine(s) and bring the rotor to operational RPM. The required torque rises from zero RPM to operating RPM at which point the rotor requires continued baseline torque with substantial overhead.

extremely different operating environment and envelope

Making a helicopter diesel electric will require installation of a very heavy generator, shaft brakes/resistor banks, heat exchanges, switchgear, flywheel plus several hundred kilograms of copper wiring and electric motors.  All in addition to the primemover(s), none of which has a pay off in terms of capability. The additional components consume valuable internal space and add significant weight far in excess of the very capable existing mechanical links which in multi-engine helicopters cannot be eliminated.

Complex, heavy and eats internal volume with no advantages

Another key consideration for railway locomotives is that weight is a good thing, since it improves traction but doesn't take much extra energy to carry around on wheels.  By contrast, weight in a helicopter is a bad thing--every extra pound of engine that doesn't improve performance will cost a pound of payload.

Yes absolutely the mass of a locomotive helps increase the maximum force that that can be applied to a wheel before it slips which actually aids acceleration and braking.  Which are somewhat counter intuitive and aren't an issue as far as capability of a locomotive since space and weight aren't a problem and nor are there any implications to forcing feeding air to the heat ex changers, inter coolers and the load banks to sink current from brakes.   High energy is space and weight consuming, something helicopters are sensitive too.

Interesting, I didn't realise that a heavy helicopter was about as powerful as a locomotive!

One thing I don't see addressed yet in other answers:
The reason for diesel-electric systems is that diesel engines (in fact, all ICE engines) are only efficient at certain RPM values. For road vehicles that accelerate and decelerate a lot, this means that most of the time, your directly linked engine is not working efficiently. To alleviate this problem, you can either have a fancy gearbox (for example, dual-clutch or CVT - or even a normal gearbox will do), or have an generator and an electric motor: this is a very handy 'gearbox' since electrons are so easy to manipulate. Some (prototype) hybrid cars also use this system.
Helicopters almost invariably operate at a fixed RPM set by the governor. This means that whatever engine is employed, you can just let it run at the best RPM using a simple fixed geared transmission. Actual variations in lift are done with blade pitch, not motor RPM. The whole point of having an electric system, (which is just a very fancy transmission like some sort of CVT), is then negated, which is why this system will never be used in both propeller fixed-wings and helicopters alike.

The same thing can be said of diesel-electric trains. I'm sure the engine RPM of a cruising train is nearly constant too. The real question is, is a typical turboshaft engine most efficient at the RPM needed for a helicopter? If so, then yes the diesel-electric idea loses some incentive.

I agree with DrZ214, this sort of system is already being studied and used in fixed-wing aircraft (which are not all fixed RPM). I don't think your point is completely wrong, I just don't agree with all of the assertions.

@fooot, the point of hybrid is that there is a battery in the circuit. Much easier in fixed-wing, since they need lower thrust/weight ratio.

@JanHudec yes, but the last sentence seems to lump all propeller/helicopter vehicles together saying they will never use it, which is where I disagree.

@DrZ214. cruising train has nearly constant RPM. But trains have friction bearings (because they are too heavy for roller bearings) lubricated with axle-driven pumps. So when the train stops, oil pressure drops, the oil gets squeezed out and the static friction is much higher than rolling. So train engine needs high torque from standstill and that is what electric engines are great at.

Only thing I miss in this interesting discussion: a helicopter transmission transcends being a 'simple gearbox'. Is has to have very high ratio (at least in turboshaft helis) plus there's a need for a 90 degrees direction change. Being a critical part it is also quite heavy and expensive. So there is a reason to look out for some alternative, even though electromechanical might not meet the demands. Hydraulics maybe?

Note the load characteristics of a helicopter and a locomotive are vastly differently, a helicopter has high torque demands as it comes to the rotor speed required to enter flight,.   Where as a the torque required by a locomotive drops away as it reaches a higher speed where the required torque is small compared to getting it rolling.
The main rotor of a helicopter is designed for operation as a maximum speed, usually related to tip speed trade-offs?  Maintaining that operating RPM is a very high amount of maximum torque available. It doesn't really drop once entering cruise flight.

@JanHudec Whoa I never knew that about trains. I'm not even sure if I even saw your comment until now. I wish there was a Trains.SE so I could ask you all this stuff.

Advantages of using electric drivetrain with diesel engines are:

High torque from zero RPM. This is extremely important for trains, because trains have much higher rolling friction in the first few feet before the wheel-driven oil pumps manage to sufficiently lubricate the bearings. Diesel trains that use mechanical drive train even often have additional short-term power sources like steam turbines to help with that.
Note that this reason does translate even to cars, which use diesel engines, but with mechanical drive trains. And for helicopters, they don't even need a gear box, because they run the rotor at constant RPM anyway.
Batteries can be easily included. This is important for submarines, because they can't run diesel engine below periscope depth, because it needs a lot of oxygen (at periscope depth they can use snorkel). But this is specific to submarines.
Other vehicles may have use for that as well, for example hybrid cars use it to get higher peak power with smaller engine that can run more efficiently. But helicopters need fairly constant power, so they don't have much use for that either.

So helicopters with diesel engine are possible, and at least Airbus Helicopters are working on it, but the electric part simply has no use. They did not exist so far, because diesel engines were too heavy, but they are becoming light enough that they should be usable in future.

When you say "they are becoming light enough", are you talking about diesel engines like those in a truck, like a piston engine? Or are you talking about a turboshaft engine running on diesel fuel?

@DrZ214, “Diesel engine” means compression-ignition reciprocating engine and that is what I am talking about. Turbines running on diesel fuel would not be noteworthy, because they all do—jet fuel is basically the same thing as automotive diesel with stricter limits for purity (and aviation diesel engines usually run on Jet-A/Jet-A1).

@JanHudec: Jet fuel is kerosene, not diesel.

@Sean, standard automotive diesel fuel is slightly heavier fraction than Jet-A1, but the ranges overlap and neither engine cares too much anyway.


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How is auto-rotation different in coaxial rotor helicopters?
Coaxial Helicopters have two main rotor sets, one on top of the other, spinning in different directions to eliminate the need for a tail rotor. I've heard it said they're also more maneuverable, all else being equal.
So how is auto-rotation different (if anything) for these kinds of helicopters? Is it any easier? Do you get longer range and/or slower (safer) descent speed?

I really hope that you get an authoratative answer to this but I doubt that we have anyone who's flown a co-axial on the site.  My guess, I can see no reason why it would be any different to any other helicopter.  Do you have a specific reason to think that it might be?

@Simon Yes, sort of, because I've heard coaxes are more maneuverable (emphasis on 'heard'). I know coaxes are more efficient because all engine power goes into lift (no tail rotor, which wikipedia says can draw up to 30% of engine power), plus the contra-rotating blades can recover some of the sideways-swept air from each other and push it down instead of sideways. It's too bad coaxes are so rare, they seem fascinating and worthy of competing with conventional helis.

Continuing with the guess work - more efficient means nothing in an auto-rotation, since there is no engine power to be lost, and sideways swpet air does not apply since the relative airflow is from under the disc and drives the inner region rather than the outer.

You might be able to answer your own question through a web search: http://www.kamov.net/kamov-net/coaxial-rotor-autorotation/

@Simon: I have flown a Kamov 26 briefly (from the right seat), but did never auto-rotate one. But I guess this is simple physics: If the inertia of the rotors is the same, autorotation should be very similar.

There's one major difference between a coaxial helicopter and a conventional single main rotor -- single tail rotor helicopter during autorotation: yaw control.
A conventional helicopter has a main rotor which produces a torque. The tail rotor thrust, applied at the end of the tail boom, provides a counter-acting "anti-torque" to keep the helicopter pointing when the pilot wants it.  As the pilot, when I change the collective pitch of the main rotor, to go into a climb or a descent, the torque required to drive the main rotor goes up or down, and the pilot counters by raising or lowering the tail rotor collective to change the tail rotor thrust to match.  Additionally, if I want to yaw the nose to the left or right, I can use the pedals to raise or lower the tail rotor thrust in order to temporarily upset the balance between the main and tail rotor torques and steer the aircraft.
Now, consider how yaw control works for a coaxial helicopter.  The torque of the upper rotor is now counter-balanced by the torque of the lower rotor.  To yaw the helicopter, the pedal inputs are set up to upset the balance of torques by raising the torque on one rotor (by increasing the collective) and lowering it on the other (decreasing collective), while maintaining the same thrust.
Now, when I increase my rate of descent to autorotation, the engines are no longer providing torque to drive the main rotor--the rotor is being driven by the relative wind.  In a conventional helicopter, the tail rotor is still being driven by torque transmitted from the main rotor through the drive shafts--so while there's no main rotor torque to balance, I can still use the pedals to change the tail rotor thrust and control my yaw.  However, in a coaxial helicopter, when the  torque on the rotors is low, differential collective does not have very much control authority: depending on the collective setting, pedal inputs can have a weak effect, no effect, or most surprisingly, a negative effect!  To counter-act this, most coaxial helicopters have very large vertical fins to provide good low-speed directional stability and many have rudders to augment control.
For instance, here's a picture showing the empennage of the Kamov Ka-25 (from aviastar.org).

More modern machines also have control systems that reverse the effect of pedal inputs on differential collective at the very low collective stick settings where control reversal can occur.  Flight manuals for coaxial helicopters advise against reducing forward speed too quickly during autorotation, since the directional stability is provided primarily by the body and tail.

Practically speaking, I have to disagree with the disadvantage of yaw control (or lack thereof). One of the main reasons a traditional helicopter has to auto-rotate is a loss of tail rotor effectiveness. So all that yaw control that a tail rotor provides is gone. AFAIK, you're supposed to fly forward during auto-rotate no matter what kind of helicopter it is, and both types will benefit from some kind of tail extension or rudder or weathervane to help keep yaw stable. So it can't really be a disadvantage of one under the other when both types usually undergo the same loss of yaw.

@DrZ214 So all that yaw control that a tail rotor provides is gone.  Nope.  Just entering autorotation will restore tail rotor authority.  Once in auto, you have full yaw control.


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Why do jet engines take a long time to throttle down?
Apparently jet engines can take several seconds, up to ten or fifteen seconds, to go from full power to idle.
Why is this? Can't a simple fuel valve shut off fuel flow? Or reduce it to whatever flow rate corresponds to idle? Either of these, if done with a valve, should happen almost instantly as far as I can tell.
This would surely cut thrust, regardless if the engine still spins for a while. Is there something dangerous about this idea? To me it seems like a fuel valve can close and open again as many times as it wants.
Note: I'm interested in turbojets and turbofans. And I care about throttling down only.

To prevent compressor surge. See (maybe duplicate): Why do turbine engines take so long to spool up?.

Inertia. But thrust will drop immediately if the fuel supply is cut. No heating, no thrust.

@mins I read that but I don't see how a surge could happen when throttling down suddenly. Maybe the opposite could happen, a sudden drop in pressure, but that doesn't sound dangerous like a spike in pressure is.

See 8.2.6 Emergency shutdown of Gas Turbine Performance.

Maybe just inertia of the rotating turbine, compressor disks

If you drop the fuel flow down in a massive step, say from 100% thrust to idle, then you run the risk of the residual air flow through the burners being far too much for the sustained burning at the lower fuel flow rate, leaning the mixture out and causing the engine to flame out.  You can't apply brakes to the fan or core, so the reduction in air flow has to happen naturally through loss of inertia in those moving components.

@PeterKämpf Can you explain "inertia" any better? Note I'm interested in cutting thrust immediately, not necessarily cutting speed. I know the airplane's inertia will keep it flying forward for a while. I'm sure the engine will still spin too because of rotational inertia. But your last sentences seem to agree that we can cut thrust right away despite all that.

@Moo That's okay. Flame out will cut thrust too. All I care about is cutting thrust, either to idle or completely zero. Of course it would be nice if the engine is undamaged and restartable in the normal way, even if normal restart procedures can take a while. As a curious aside, however, I can't see why it would flame out in your scenario. An overabundance of oxygen mixed with fuel, however small, should not cause a fire to die. As you probably guessed I don't know much about the gas dynamics in engines so hopefully someone could explain why the flame would die.

@DrZ214 the combustion would die because the reactions between the fuel and the oxygen become too spread out, meaning you either have to have a constant ignition source or the combustion ceases because it can't propagate.  A flame is basically a chain reaction, with the energy released by one fuel-oxidiser reaction triggering a reaction in a neighbouring fuel-oxidiser pair, but move those pairs far enough apart and the energy is no longer enough to trigger the reaction.  This is why fire spreads where there is both a fuel source and an oxidiser.

@DrZ214 of course, all of this is ignoring that in modern high bypass engines, the bulk of thrust (like 80% or more) is generated by the movement of air by the fan at the front - to cut that thrust to zero immediately, you need to stop that fan or gear the fan blades so they can be feathered.  Stopping the fan means a hefty brake (those fans weigh tonnes and have a high RPM) and feathering the fans means more weight and complexity.  Airlines don't want either of those, so it hasn't ever been a problem that has needed to be solved.

@Moo Okay then it sounds like what I want is a thrust reverser, or maybe a thrust deflector to vector the air in two opposite sideways directions, or even some kind of bleed air opening. PS your comment would make a great answer.

@DrZ214 most jet engines have thrust reversers already :)  Often they are used to enhance braking performance on landing, but they are also used to deflect spool-down thrust on landing as well when deployed at engine idle.

I guess the shut down must be done in a controlled fashion to accomodate for differential cooling patterns of each component. The blades cool at a rate way different than the engine nacelle and casing.

@PeterKämpf: While inertia will keep the shaft & everything attached spinning for a short while, the air passing through the engine does serve as a massive brake. Obvious, from an energy point of view : the spinning engine contains only a fixed amount of energy, and accelerating air through the engine drains that energy. High RPM means a lot of energy, but also a lot of air being accelerated, trust produced while the engine spools down, and thus energy carried away.

@Moo another way to stop the fan quickly is to use the electric generator in reverse. Large commercial jet engines have a generator that supplies electric power to the cabin. If I'm not mistaken, it is mechanically simple to reverse this process (take electric power to brake the spinning turbine). But then the only question is, where will you get the power from to do that (since we just shut off the engines)?

@DrZ214 even more important than where the energy comes from would be whether the generators can support the load - you can't simply turn a generator into a motor as they are designed for different loads, and as it will already be doing electrical energy production then regenerative braking is out as it won't add anything to it.  Also bear in mind that not all engines may have generators.

The simple answer is rotational inertia, once you get a heavy mass spinning fast that heavy mass will have a tendency to continue spinning fast unless something else is acted upon the object. Piston engines have a great deal of friction between the piston walls and pistons (how they hold the explosion in) this helps them slow down faster than a jet in which the turbine is made to easily handle this thus the joints are very well made and very free moving. While cutting fuel will keep the engine from continuing to generate thrust the turbine will continue to spin until a force causes it to stop (drag and friction really).

Jet engines have little mechanical margin, and operate at high temperatures. That means all components expand by more than the mechanical margins when heated. This is no problem when everything expands and contracts together in a controlled fashion. This may be counterintuitive but is easily demonstrated. Cut a circular hole in a metal plate, and heat up the plate. The hole will grow with the same expansion rate as the surrounding material.
Now what happens if we suddenly stop the fuel flow? The compressive heating of air flowing into the engine doesn't suddenly stop, but all the air flowing through the engine does cool it. This means that the turbine in the back does cool as relatively cool air flows through it. Not only does this cause thermal stresses, but the turbine will shrink.

Are you saying that even if the throttle control is pulled to idle, some ECU will continue to feed fuel into the engine to avoid rapid cooling?

Basically it is that the more weight the moving object has (the turbine in this case) the more time it takes to slow down because of the energy stored in a moving object. It is the same in all object which have a large mass. In an automobile, there is the flywheel which stores some of the energy the engine produces so that a little hiccup in the pistons would not interrupt the spinning. With the jet engine, the large mass of the turbine also keeps the increase or decrease of speed at a constant rate so that the engine would not produce "jerks" of power when the throttle is changed.

This is a start of an answer; can you expand on it?